The present invention relates to computed tomography, and more particularly, to a method and apparatus for producing time resolved angiograms using a computed tomography (“CT”) system.
In a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “image plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce what is called the “transmission profile”.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. The transmission profile from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Angiography is a diagnostic modality concerned with diseases of the circulatory system. Many imaging modalities are now available for researching vascular structures, including ultrasound, computed tomography, and magnetic resonance imaging. One of the most popular imaging modalities for angiography is digital subtraction angiography (DSA). In DSA, a pre-injection image (or mask) is obtained, a contrast agent is injected, and a series of images are acquired as the contrast agent flows into the vascular structures. The mask image is subtracted from the contrast enhanced images to remove background tissues and provide high contrast in vascular structures.
Although a number of angiographic imaging modalities are available, all of these known methods suffer from one or more disadvantages including sensitivity to artifacts from patient motion, low signal to noise ratio, and the requirement for a significant load of contrast agent to be inserted in the patient. Furthermore, although DSA had been developed with the hope of using it to perform intravenous contrast imaging, which is less invasive and less uncomfortable for the patient than arterial injection, attempts to provide such a system have been generally unsuccessful.
In Computer Rotation Angiography (CRA), a computed rotational angiography system such as that described by Fahrig, Lownie and Holdsworth (Use of a C-Arm system to generate True 3D Computed Tomography Rotational Angiograms; Preliminary in vitro and In vivo Results. R. Fahrig, S. Lownie, and D W Holdsworth, AJNR 18:1507–154, September 1997) is employed to acquire a series of three dimensional images during the uptake of a contrast agent. Because it is desirable to acquire the three-dimensional data sets obtained using this apparatus, as quickly as possible in order to provide a high time resolution during the dynamic study, only 120 projection angles, or views, are acquired. This is significantly less than that demanded by the Nyquist sampling theorem. Therefore, the angiogram reconstructed from a single data set contains streak artifacts. These streak artifacts preclude the use of this CRA method for intravenous angiography because of the reduced vasculature contrast provided by this contrast injection method.